Solid state lighting apparatus are used for a number of lighting applications. For example, solid state lighting panels including arrays of solid state light emitting devices have been used as direct illumination sources, for example, in architectural and/or accent lighting. A solid state light emitting device may include, for example, a packaged light emitting device including one or more light emitting diodes (LEDs). Inorganic LEDs typically include semiconductor layers forming p-n junctions. Organic LEDs (OLEDs), which include organic light emission layers, are another type of solid state light emitting device. Typically, a solid state light emitting device generates light through the recombination of electronic carriers, i.e. electrons and holes, in a light emitting layer or region. A solid state light emitting device typically emits light having a specific wavelength that is a characteristic of the material(s) (e.g., semiconductor material or materials) used in the light emitting layer or region. Stated in other words, solid state light emitting devices are typically monochromatic.
The color rendering index (CRI) of a light source is an objective measure of the ability of the light generated by the source to accurately illuminate a broad range of colors. The color rendering index ranges from essentially zero for monochromatic sources (e.g., semiconductor light emitting diodes) to nearly 100 for incandescent sources. To improve color output, a solid state light emitting device that generates light having a first wavelength (e.g., blue light) may be combined with a phosphor that converts a portion of the light emitted by the solid state lighting device (having the first wavelength) to a second wavelength (e.g., yellow light), and light having the first and second wavelengths may be combined. For example, a yellow phosphor may be provided with/on a light emitting diode emitting blue light to provide a blue-shifted-yellow (BSY) light source. Light generated from such phosphor-based solid state light sources, however, may still have relatively low color rendering indices.
It may be desirable to provide a lighting source that generates a white light having a high color rendering index, so that objects and/or display screens illuminated by the lighting panel may appear more natural. Accordingly, to improve CRI, red light may be added to BSY light generated by a blue LED and a yellow phosphor, for example, by adding red emitting phosphor and/or red emitting devices to the apparatus. Other lighting sources may include red, green and blue light emitting devices. When such combinations of light emitting devices are energized simultaneously, the resulting combined light may appear white, or nearly white, depending on the relative intensities of the red, green and blue sources.
In a lighting apparatus providing directed illumination, a plurality of light emitting devices having different chromaticities may be arranged so that light emitted thereby is combined to provide a combined optical output. Moreover, the light emitting devices may be configured in/on the lighting apparatus to provide that the optical output has one or more of a desired color, dominant wavelength, CRI, correlated color temperature (CCT), etc., and/or to provide that the optical output is not significantly diffused. In such apparatus, there continues to exist a need for control of uniformity of the optical output over expected ranges of operating temperatures.